As researchers have moved beyond the NFL to look at the brains of high school and college athletes, they’re increasingly finding signs of traumatic brain injuries in young football players.

Hilinski’s parents said on “Today” that their son was diagnosed with chronic traumatic encephalopathy, or CTE, a neurodegenerative disease caused by repetitive head trauma. The Washington State University quarterback killed himself early this year.

The disease, which can currently be diagnosed only after death, is the most serious condition on a list of impairments and diseases linked to repetitive head trauma.

It was diagnosed after death in an 18-year-old high school athlete whose family donated his brain to Boston University for study. The boy suffered multiple concussions while playing football, and his brain was the earliest evidence of CTE yet found in a human brain.

A 2015 Mayo Clinic study analyzing the donated brains of athletes who played contact sports in high school found CTE in one-third, compared to none in the non-athlete population. That was true even though 33 people out of the 198 in the non-athlete group had a documented one-time brain injury from causes including falls, motor vehicle accidents and assaults.

The telltale sign of CTE is a buildup of tau proteins in brain tissue, which cause the brain to shrink. That buildup can only be seen by cutting into the brain, hence the postmortem diagnosis.

Tau proteins are part of the structure of axons, helping to hold small, tube-like structures together. Repetitive blows to the head cause them to detach from the axons and break, forming tangled masses.

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Dr. Bennet Omalu, the pathologist who first discovered CTE in the brain of former Pittsburgh Steelers center Mike Webster, has since become an advocate for reforms in youth sports to limit contact and prevent brain damage.

Omalu has reported receiving calls from the parents of middle and high school athletes whose families are noticing changes in behavior following blows to the head. Many eerily mirror the symptoms of retired NFL players: difficulty focusing, increased aggression and irritability, and poor performance in school.

Though early discussion about head trauma in football centered around concussions, more research has shown that repetitive blows to the head, even if they don’t cause any symptoms at the time, may be the culprit. And head trauma is common in youth sports outside of football, including wrestling and soccer.

A concussion is a very broad term for any blow to the head or violent shaking of the head followed by symptoms like a headache, dizziness, nausea or memory problems. A “sub-concussive injury” is a blow to the head where no symptoms follow.

Boston University has become the leading national center on CTE research, with a brain bank program that examines the brains of former athletes and military members. A study published in January by BU researchers suggested CTE was linked to repetitive blows to the head, not to concussions.

Tyler Hilinski cemented his place in Washington State lore in September with a 22-yard touchdown pass to Jamal Morrow in double overtime to claim a 47-44 win over Boise State. | Read more »

Researchers induced head trauma in mice and observed their response, and also examined the brains of several young athletes who had died while recovering from head injuries. They found that early signs of CTE in the brain may be present even without outward signs of concussion following injury.

“The same brain pathology that we observed in teenagers after head injury was also present in head-injured mice,” said Lee Goldstein, a BU School of Medicine associate professor of psychiatry, in a news release about the study. “We were surprised that the brain pathology was unrelated to signs of concussion.”

About 20 percent of athletes who show early symptoms of CTE have never had a diagnosed concussion, BU says.